10/31 2:30 PM
Dr. Lingsen Meng (UC Berkeley)
The 2013 Okhotsk Deep-Focus Earthquake: Thermally-Controlled Rupture Near the Edge of a Slab "
A exceptional deep-focus earthquake occurred on May 24th 2013, 610 km beneath the Sea of Okhotsk. The M 8.3 quake surpassed in magnitude the M 8.2 1994 Bolivian earthquake, rendering it the largest deep-focus earthquake (focal depth > 300 km) ever recorded by modern seismology. The earthquake offers a special opportunity to address open questions about the physical mechanisms of deep-focus earthquakes. Broadband (16 s to 4 Hz) back-projections of the teleseismic seismograms recorded by the North American and European seismic networks indicates bilateral rupture towards NE and SSE. By considering direct P and pP phases separately, we find that a single sub-horizontal fault-plane is consistent with the source images obtained from both phases.The SSE rupture propagates a long distance across the slab, implying that physical mechanisms other than or in addition to phase transformation are active during deep earthquake ruptures, e.g. thermal shear instability. The rupture speed for both branches is about 60% on average of the shear-wave speed (3.5 km/s) which is compatible to the overall fast rupture speed of deep earthquakes at this depth but significantly faster than the Bolivian earthquakes. While the NE rupture is only visible at high frequency, the SSE rupture is consistently observed across a broad frequency band. This frequency-dependent rupture mode can be explained by a lateral variation of the rise time controlled by the variable thermal state of the slab near its northern end.